Omit tvg-id attribute when EPG column is empty instead of emitting tvg-id=""

42 channels across 14 lists have an EPG column in their markdown table row
but the column is blank. The condition in Channel.to_m3u_line() only checked
for None (no EPG column at all) but not for an empty string, so these
channels got a tvg-id="" attribute in the generated playlist. An empty
tvg-id tells IPTV players "this channel has an EPG ID and it is the empty
string", which prevents EPG matching rather than falling back to name-based
matching the way a missing attribute would.

Changing `if self.epg is not None` to `if self.epg` makes both None and
empty string omit the attribute, matching the existing pattern used by
the chno and country_code attributes on the lines just above.

make_playlist.py

@@ -123,7 +123,7 @@ class Channel:  # pylint: disable=too-few-public-methods,too-many-instance-attri
         """Render this channel as a #EXTINF entry followed by its URL."""
         country = f' tvg-country="{self.country_code}"' if self.country_code else ""
         chno = f' tvg-chno="{self.chno}"' if self.chno else ""
-        epg = f' tvg-id="{self.epg}"' if self.epg is not None else ""
+        epg = f' tvg-id="{self.epg}"' if self.epg else ""
         # Strip trailing in-list markers (Ⓖ/Ⓢ/Ⓨ/...) from tvg-name so EPG
         # matching isn't broken by a symbol meant for human readers.
         tvg_name = re.sub(r'\s*[Ⓐ-ⓩ]+\s*$', '', self.name)